Method for nitriding metal in salt bath and metal manufactured using the same

ABSTRACT

Provided is a method for nitriding a metal in a salt bath by using a non-cyanide salt and a nitrided metal manufactured using the same. The method includes the steps of: immerging at least one salt selected from the group consisting of KNO 3 , KNO 2 , Ca(NO 3 ) 2 , NaNO 3  and NaNO 2  into the salt bath; melting the salt by heating and maintaining the molten salt at a predetermined temperature; and submerging the metal in the salt bath. Nitriding in non-cyanide salts, such as potassium nitrate (KNO 3 ), potassium nitrite (KNO 2 ), sodium nitrate (NaNO 3 ), sodium nitrite (NaNO 2 ), calcium nitrate (Ca(NO 3 ) 2 ) and their mixtures, is capable of solving an environmental pollution problem and reducing a cost. Also, the method is capable of increasing nitrided depth of the metal two to six times compared to conventional nitriding methods. As a result, the method can be carried out in various application fields.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for nitriding metal in a saltbath and nitrided metal manufactured using the same; and, moreparticularly, to a method for nitriding iron or steels by usingnon-cyanide salt bath, and nitrided iron or steels manufactured usingthe same.

2. Background of the Related Art

Steels have been widely used for machine parts because of their inherentproperties. To be used for machine parts, steels are usually firstheat-treated to impart thereto strength, toughness and durability, allof which are the qualities machine parts require. In addition, formachine parts that are often exposed to corrosive environment, surfacesthereof are further heat-treated to impart thereto corrosion resistance.

Nitriding is one of the methods for processing the metal surface toimpart thereto a corrosion resistance thereof. The nitriding methodsinclude gas nitriding using NH₃ gas, salt bath nitriding using KCN, KCNOetc., gas nitrocarburizing (carbo-nitriding) using a mixture of NH₃ gasand RX gas, i.e., endothermic gas, and ion nitriding involving aninsertion of a mixture of N₂ and H₂ gas into plasma.

Generally, although nitriding is applied to steels to improve theirabrasion(wear) resistance and fatigue resistance, it can also be carriedout to improve the corrosion resistance thereof.

Of the nitriding methods mentioned hereinabove, the salt bath nitridingis most widely used for a variety of machine parts including automobilecomponents, because properties of chemicals for the salt bath and theirmelting points can be freely controlled to provide stability through awide range of process temperatures without eroding the surface of theobject being treated. To be more specific, in addition to its excellentthermal conductivity, soaking properties and easily controllableprocessing conditions, it is cheaper to design and maintain, comparedwith other nitriding methods. For example, it is easy to operate thesalt bath, and the heating rate is 4 times faster in the salt bath thanin the atmosphere. The salt bath is especially suitable toheat-treatment of high speed steel which is sensitive to crystal(grain)growth. When a material treated in a salt bath comes into a contact withthe atmosphere, a film including the salt bath constituents is formed onthe surface thereof, the film preventing oxidation by preventing thematerial from a direct contact with the atmosphere. Furthermore, thesurface of the material thus treated is rather clean, making the saltbath an ideal heat-treatment for both mass production andsmall-lot-sized production.

Cyanide-containing salt is generally used for a salt bath nitridingmethod, producing cyanide ions inside the bath. Since the cyanide ion isclassified as a toxic chemical, it must be carefully and tightlycontrolled, and this can be an expensive proposition. Also, there is aproblem of a cost involved for processing waste water and gas.

Further, the nitriding treatment in a molten salt including cynides is anitrocarburizing (carbo-nitriding) method involving a simultaneouspenetration of carbon and nitrogen. It has a shortcoming in thatalthough the surface hardness of the material thus treated improvessignificantly, the tensile strength gets only slightly enhanced. Theconventional salt bath nitriding method using a cyanide salt also has aproblem that its applications are limited to molds or gears since thedepth to which the material can be nitrided is limited.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a methodfor nitriding a metal using non-cyanide salts, and a nitrided metalmanufactured using the same.

It is another object of the present invention to provide a salt-bathnitriding method for nitriding a metal, in which nitrogen penetratesinto the metal, and a nitrided metal manufactured using the same.

It is yet another object of the present invention to provide a salt bathnitriding method for nitriding a metal, capable of increasing hardnessand tensile strength of the metal to be treated, and a nitrided metalmanufactured using the same.

It is still another object of the present invention to provide a saltbath nitriding method for nitriding a metal, capable of maximizing anitriding depth, and a nitrided metal manufactured using the same.

In accordance with one aspect of the present invention, there isprovided a method for nitriding a metal in a salt bath, the methodincluding the steps of: a) immersing a non-cyanide salt into the saltbath; b) melting the salt by heating and maintaining the molten salt ata predetermined temperature; and c) submerging the metal in the saltbath.

In the present invention, it is preferred that the non-cyanide saltincludes at least one selected from a group consisting of NaNO₃, NaNO₂,KNO₃, KNO₂ and Ca(NO₃)₂, and the metal is one of iron and steels.

At this time, the predetermined temperature is within a range of 400° C.to 700° C., and the submerging time is within a range of 1 minute to 24hours.

In the present invention, when iron is nitrided in the salt bathincluding at least one of the group consisting of KNO₃, KNO₂, Ca(NO₃)₂,NaNO₃, and NaNO₂, the iron can be nitrided into a depth of 0.1 mm to 3.0mm from its surface.

In the present invention, when a steel is nitrided in the salt bathincluding at least one of the group consisting of KNO₃, KNO₂, Ca(NO₃)₂,NaNO₃, and NaNO₂, the steel can be nitrided into a depth of 0.1 mm to3.0 mm from its surface.

The steel includes ultra-low carbon steel, low carbon steel, mediumcarbon steel, high carbon steel, alloy steel and IF steel.

The ultra-low carbon steel nitrided by the present invention has thesurface hardness ranging from more than 120 Hv to equal to or less than450 Hv. The low carbon steel has the surface hardness being more than200 Hv to equal to or less than 410 Hv. The medium carbon steel has thesurface hardness being more than 130 Hv to equal to or less than 420 Hv.The high carbon steel has the surface hardness being more than 150 Hv toequal to or less than 400 Hv. The alloy steel has the surface hardnessbeing more than 200 Hv to equal to or less than 410 Hv. IF steel has thesurface hardness being more than 165 Hv to equal to or less than 400 Hv.The surface hardness of the steels nitrided by the present invention canbe improved to a maximum of 420 Hv. The surface hardness of the ironnitrided by the present invention is also improved.

The ultra-low carbon steel nitrided by the present invention has thetensile strength ranging from more than 35 kgf/mm² to equal to or lessthan 110 kgf/mm². The low carbon steel has the tensile strength rangingfrom more than 45 kgf/mm² to equal to or less than 110 kgf/mm². Themedium carbon steel has the tensile strength ranging from more than 45kgf/mm² to equal to or less than 100 kgf/mm². The high carbon steel hasthe tensile strength ranging from more than 60 kgf/mm² to equal to orless than 95 kgf/mm². The alloy steel has the tensile strength rangingfrom more than 55 kgf/mm² to equal to or less than 110 kgf/mm². Thetensile strength of IF steel and iron can be improved by the nitridingmethod of the present invention.

The salt-bath nitriding method of the present invention can be appliedto the iron, the IF steel, the carbon steel including the ultra-lowcarbon steel having a carbon content of at least 0.0001 wt % to lessthan 0.13 wt %, the low carbon steel having a carbon content of at least0.13 wt % to less than 0.2 wt %, the medium carbon steel having a carboncontent of at least 0.21 wt % to less than 0.51 wt %, and the highcarbon steel having a carbon content of at least 0.51 wt % to less than2.0 wt %, the steel having a chrome content of 0.1 wt % to 1.5 wt %, thesteel having a molybdenum content of 0.05 wt % to 0.5 wt %, the steelhaving a nickel content of 0.1 wt % to 10 wt %, the steel having amanganese content of 0.1 wt % to 2.0 wt %, the steel having a boroncontent of 0.001 wt % to 0.1 wt %, the steel having a titanium contentof 0.01 wt % to 0.1 wt %, the steel having a vanadium content of 0.05 wt% to 0.15 wt %, the steel having a niobium content of 0.005 wt % to 0.1wt %, and the steel having an aluminum content of 0.005 wt % to 0.1 wt%. Also, the salt-bath nitriding method of the present invention can beapplied to the alloy steel including at least two kinds of the steelssuggested above.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of the preferredembodiments given in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a graph illustrating relationship between a nitriding time anda hardness profile in a steel nitrided in accordance with a firstembodiment of the present invention;

FIG. 2 is a graph illustrating relationship between the nitriding timeand the hardness profile in the steel nitrided in accordance with thefirst embodiment of the present invention;

FIG. 3 is a graph illustrating relationship between a nitridingtemperature and the hardness profile in the steel nitrided in accordancewith the first embodiment of the present invention;

FIG. 4 is a graph illustrating relationship between the nitriding timeand the surface hardness of the steel nitrided in accordance with thefourth embodiment of the present invention;

FIG. 5 is a graph illustrating relationship between the nitridingtemperature and time and the hardness profile in the steel nitrided inaccordance with the fourth embodiment of the present invention;

FIG. 6 is a graph illustrating relationship between the nitriding timeand the hardness profile in the steel nitrided in accordance with thefourth embodiment of the present invention;

FIG. 7 is a graph illustrating the hardness profile in the steelnitrided in accordance with the fifth embodiment of the presentinvention;

FIG. 8 is a graph illustrating the hardness profile in the steelnitrided in accordance with the sixth embodiment of the presentinvention; and

FIG. 9 is a graph illustrating relationship between a mixture ratio of amixed salt and the hardness profile in the steel nitrided in accordancewith the sixth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in more detail.

In nitriding of a metal, the present invention incorporates therein thenitrogen dissolution principle involving a non-cyanide molten salt, moreparticularly, NaNO₃, NaNO₂, KNO₃, KNO₂, Ca(NO₃)₂ and mixtures thereof asa molten salt, as opposed to a conventional nitriding method such as anitrocarburizing (carbo-nitriding) method involving the use of cyanides,e.g., KCN and NaCN, as the molten salt wherein carbon and nitrogen aresimultaneously diffused into the metal.

The method for nitriding the metal in accordance with the presentinvention involves immersing at least one salt from a group consistingof NaNO₃, NaNO₂ KNO₃, KNO₂ and Ca(NO₃)₂ into a salt bath, melting thesalt and maintaining of the molten salt at a predetermined temperatureranging from 400° C. to 700° C.

Subsequently, the metal to be nitrided is submerged in the bath for 1minute to 24 hours.

During this time, nitrogen, oxygen and nitrogen oxides are generatedfrom the non-cyanide molten salts of the present invention, NaNO₃, NaNO₂KNO₃, KNO₂, Ca(NO₃)₂ and mixtures of thereof, by the following reactionformulae 1 to 3.

The following reaction formula 1 represents nitrogen formation reactionin the molten salt bath of NaNO₃ and NaNO₂.

NaNO₃→NaNO₂+½O₂

2 NaNO₂→Na₂O+NO₂+NO

2NaNO₂+2NO→2NaNO₃+N₂  [Reaction formula 1]

The following reaction formula 2 represents nitrogen formation reactionin the molten salt bath of KNO₃ and KNO₂.

KNO₃→KNO₂+½O₂

2KNO₂→K₂O+NO₂+NO

2KNO₂+2NO→2KNO₃+N₂  [Reaction formula 2]

The following formula 3 shows nitrogen formation reaction in the moltensalt bath of Ca(NO₃)₂.

Ca(NO₃)₂→CaO+2NO₂+½O₂

2NO₂→2O₂+N₂  [Reaction Formula 3]

As shown in Table 1, those metals nitrided, including carbon steel(including ultra-low carbon steel, low carbon steel, medium carbon steeland high carbon steel), alloy steel, IF steel and iron using thesalt-bath nitriding method in accordance with the present invention arenitrided to a depth of 0.1 mm to 3.0 mm from the surface. The range ofnitrided depth/diffusion layer thickness obtained through the presentinvention is 2 to 6 times larger than that obtained using theconventional nitriding methods, meaning that a nitrided/diffusion layerformed using the nitriding method of the present invention extends fromthe surface to the metal inner part, and consequently the surfacehardness and tensile strength of the metal also improve compared tothose of the metal nitrided using the conventional nitriding method.Reference for the table 1 are as follows:

K. Funatani, “Low-Temperature Salt Bath Nitriding of Steels”, MetalScience and Heat Temperature, Vol. 46, No. 7, PP. 277-281 (2004).

TABLE 1 Thickness of Temperature diffusion Nitriding method (K) Type ofSteel layer (μm) Nitride process by 953 Low carbon 3000 the presentsteel invention 913 IF steel 1500 Tufftride TFI 853 1015 800 853 1045780 853 34Cr4 480 853 X210Cr12 160 Tufftride NSI 843 1015 780 843 SCM435171 “Soft” Nitriding in 843 SS2250 353 gas medium “Soft” Nitriding in793 38CrMoAl 78–97 gas medium — 40Cr 63–80 Gas Nitriding 773 SAE9254 49Plasma Nitriding 793 722M24 72 (Pused) 793 (DC) 722M24 Plasma Nitriding833 En40B 100 813 En19 110 793 Nitraps 46 823 36CrMo 100 793 36CrMo +0.1Y 200 823 36CrMo + 0.1Ce 215 Low-temperature 753 SKD61 150 salt bathNitriding 843 SKD61 106 (palsonite) 753 SCM435 141 843 SCM435 200

Hereinafter, embodiments of the present invention will be described indetail with reference to the attached drawings.

First Embodiment

In accordance with the first embodiment of the present invention, steelis nitrided using the NaNO₃ molten salt. The nitrided steel includesultra-low carbon steel, low carbon steel, medium carbon steel, highcarbon steel and alloy steel.

Each of the ultra-low carbon steel, low carbon steel, medium carbonsteel, high carbon steel and alloy steel is submerged in the NaNO₃molten salt bath for 2 hours at a temperature of 500° C.

Table 2 shows changes in surface hardness and tensile strength of thesamples nitrided in the molten salt bath, wherein the hardness wasmeasured using a Vickers hardness tester under a load of 1 kgf.

In case of ultra-low carbon steel, the surface hardness increases by119% and the tensile strength increases by 47%. In case of low carbonsteel, the surface hardness increases by 47% and the tensile strengthincreases by 19%.

In case of medium carbon steel, the surface hardness increases by 32%and the tensile strength increases by 18%. In case of high carbon steel,the surface hardness increases by 28% and the tensile strength increasesby 16%. In case of alloy steel, the surface hardness increases by 24%and the tensile strength increases by 17%.

That is, in case of steel, the surface hardness increases by 20% to 120%and the tensile strength increases by 15% to 50%.

The differences in the amount of increases shown in the surface hardnessdepending on the steel type can be attributed to the differences in thenitrogen diffusion rate associated with each type of steels determinedby the carbon content therein.

TABLE 2 Change of Hardness Change of Tensile (Hv) Strength (kgf/mm²) In-After In- Type Before After creasing Before nitrid- creasing ofnitriding nitriding rate nitriding ing rate steel process process (%)process process (%) Ultra 128 280 119 34 50 47 low carbon steel Low 194286 47 62 74 19 carbon steel Medium 183 241 32 56 66 18 carbon steelHigh 230 294 28 73 85 16 carbon steel Alloy 226 281 24 71 83 17 steel

FIG. 1 is a graph the showing the hardness distribution in the thicknessdirection of the ultra-low carbon steel before (As) and after nitridingin the NaNO₃ molten salt bath at 500° C. for 30 minutes, 1 hour, 2 hoursand 5 hours, respectively.

The nitrided depth or the diffusion depth increases with increasingnitriding time, and the hardness decreases with increasing distance fromthe surface because the nitrogen concentration decreases with increasingdistance from the surface. When the steel is nitrided for 5 hours, itcan be seen that the steel is nitrided to a depth of about 0.6 mm fromthe surface.

FIG. 2 shows the hardness distribution along the thickness direction oflow carbon steel nitrided in the NaNO₃ molten-salt bath at 680° C. for3, 6, 12 and 24 hours, respectively, wherein the hardness is measuredusing a Vickers hardness tester under a load of 3 kgf.

As shown in FIG. 2, the nitrided depth or the diffusion depth of thesteel increases with increasing nitriding time. The nitrided depth ofthe steel after nitriding for 24 hours is about 3 mm, which is 6 timesdeeper than that obtained from the conventional nitriding method.

Also, the surface hardness after nitriding is 450 Hv, which is more than4 times higher than that of the non-treated specimen.

Accordingly, the nitriding method of the present invention can increasethe nitrided depth of the steel by 2 to 6 times compared to theconventional cyanide-based salt bath nitriding method.

FIG. 3 shows hardness distributions along the thickness direction of theultra-low carbon steel before and after nitriding in the NaNO₃molten-salt bath at 500° C. and 600° C. for 3 hours. The nitrided depthof the steel nitrided at 600° C. is 3 times deeper than that of thesteel nitrided at 500° C. The surface hardness of the steel nitrided at600° C. is 100 Hv higher than that of the steel nitrided at 500° C. Thatis, the surface hardness and nitrided depth of steel increase withincreasing nitriding temperature.

Table 3 shows changes in tensile strength of ultra low carbon steeldepending on the nitriding temperature wherein the samples are nitridedfor 3 hours at 450° C., 500° C., 550° C. and 600° C., respectively,using the salt-bath nitriding method of the first embodiment of thepresent invention.

As shown in FIG. 3, in case of the nitriding temperature of 450° C., thetensile strength increases by 5%. As the temperature increases, thetensile strength of the steel also increases. Accordingly, when thetemperature is 600° C., the tensile strength increases by 134%.

TABLE 3 Nitriding Nitriding Tensile Increasing temperature time strengthrate Division (° C.) (h) (kgf/mm²) (%) Before — — 34.8 0 nitriding After450 3 36.6 5 nitriding 500 50.8 46 550 64.5 85 600 81.4 134

That is, since it is possible to simultaneously improve the hardness andthe tensile strength by nitriding the steel according to the firstembodiment, the present invention can be applied to diverse fieldsincluding diverse components and structural members.

Second Embodiment

In accordance with the second embodiment of the present invention, steelis nitrided by using the NaNO₂ molten salt.

Steels including ultra-low carbon steel, low carbon steel, medium carbonsteel, high carbon steel and alloy steel are submerged in the salt bathat 450° C. for 2 hours.

Table 4 shows changes in surface hardness and tensile strength of thesamples nitrided in the molten salt bath, wherein the surface hardnessis measured using a Vickers hardness tester under a load of 1 kgf.

For ultra-low carbon steel, the surface hardness increases by 54% andthe tensile strength increases by 21%. For low carbon steel, the surfacehardness increases by 32% and the tensile strength increases by 15%.

For medium carbon steel, the surface hardness increases by 19% and thetensile strength increases by 13%. For high carbon steel, the surfacehardness increases by 18% and the tensile strength increases by 12%.

For alloy steel, the surface hardness increases by 17% and the tensilestrength increases by 14%.

That is, in case that steels are nitrided by the molten salt bathnitriding method of the second embodiment of the present invention, thesurface hardness increases by 15% to 60%, and the tensile strengthincreases by 10% to 25%.

Accordingly, the molten salt bath nitriding method in accordance withthe second embodiment of the present invention also increases thesurface hardness and tensile strength of the steels.

TABLE 4 Change of Hardness Change of Tensile (Hv) Strength (kgf/mm²) In-In- Type creasing After creasing of Before After rate Before nitrid-rate steel nitriding nitriding (%) nitriding ing (%) Ultra- 128 197 5434 41 21 low carbon steel Low 194 257 32 62 71 15 carbon steel Medium183 218 19 56 63 13 carbon steel High 230 271 18 73 82 12 carbon steelAlloy 226 265 17 71 81 14 steel

Third Embodiment

In accordance with the third embodiment of the present invention, steelsare nitrided using the KNO₂ molten salt.

The steels including ultra-low carbon steel, low carbon steel, highcarbon steel and alloy steel are submerged in the molten salt bath at480° C. for 2 hours.

Table 5 shows changes in hardness and tensile strength of the samplessubmerged in the molten salt bath, wherein the surface hardness ismeasured using a Vickers hardness tester under a load of 1 kgf.

For ultra-low carbon steel, the surface hardness increases by 45% andthe tensile strength is increases by 15%. For low carbon steel, thesurface hardness increases by 25% and the tensile strength increases by11%.

For high carbon steel, the surface hardness increases by 17% and thetensile strength increases by 10%. For alloy steel, the surface hardnessincreases by 12% and the tensile strength increases by 11%.

That is, when the steels are nitrided using the molten salt bathnitriding method of the third embodiment of the present invention, thesurface hardness increases by 10% to 50%, and the tensile strengthincreases by 10% to 20%.

Accordingly, the molten salt bath nitriding method in accordance withthe third embodiment of the present invention also increases the surfacehardness and the tensile strength of the steels.

TABLE 5 Change of Hardness Change of Tensile (Hv) Strength (kgf/mm²) In-In- Type creasing After creasing of Before After rate Before nitrid-rate steel nitriding nitriding (%) nitriding ing (%) Ultra- 128 186 4534 39 15 low carbon steel Low 194 243 25 62 69 11 carbon steel High 230268 17 73 80 10 carbon steel Alloy 226 252 12 71 97 11 steel

Fourth Embodiment

In the fourth embodiment of the present invention, steel is nitridedusing the KNO₃ molten salt.

The steel to be nitrided is Interstitial-Free (IF) steel, which includescarbon (C) of 0.003 wt %, manganese (Mn) of 1.23 wt %, aluminum (Al) of0.037 wt %, titanium (Ti) of 0.027 wt %, phosphorus (P) of 0.050 wt %,nitrogen (N) of 0.002 wt % and sulfur (S) of 0.008 wt %.

The IF steel is nitrided in the KNO₃ molten bath at 560° C., 580° C.,600° C., 620° C. and 640° C., respectively.

FIG. 4 shows the surface hardness of the IF steel nitrided in the KNO₃molten bath as functions of time and temperature.

As shown in FIG. 4, as the nitriding time and temperature increase, thesurface hardness increases under most temperature conditions. Althoughthe increase of the hardness can be explained as solution strengthening,the present invention is not limited to this theory.

However, when the nitriding time in the KNO₃ molten salt at 620° C.exceeds 8 hours, or the nitriding time in the KNO₃ molten salt at 640°C. exceeds one hour, the surface hardness decreases. It is understoodthat this decrease in the surface hardness is caused by the formation ofthe nitrided layer in the grain boundaries of the IF steel.

In Table 6, the surface hardness values of the IF steel nitrided by thethird embodiment of the present invention are given. When the IF steelis nitrided at temperatures of 560° C. to 640° C., the surface hardnessincreases by 75% to 130%.

TABLE 6 Change of Hardness (Hv) Change of Hardness after nitriding for16 h. (Hv) after nitriding for 1 h. Increasing Increasing NitridingBefore After rate Nitriding Before After rate Temperature nitridingnitriding (%) Temperature nitriding nitriding (%) 560° C. 165 289 75620° C. 165 336 104 580° C. 165 329 99 640° C. 165 355 115 600° C. 165379 130

FIG. 5 shows the hardness distribution along the thickness direction ofthe IF steel nitrided by the fourth embodiment of the present invention.

The IF steel is nitrided in the KNO₃ molten salt at 560° C. for 16 hoursand at temperatures of 560° C., 580° C., 600° C. and 620° C. for 8hours.

Referring to FIG. 5, the hardness of the IF steel decreases withincreasing depth from the surface because the nitrogen concentrationdecreases with increasing distance from the steel surface. When thenitrided depth is defined as the distance between the surface and theposition where the hardness value is equaled to 110% of that of thecenter of the IF steel before nitriding, the nitrided depth formed ineach condition ranges from about 1.38 mm to 1.5 mm, which is 3 to 5times thicker than the thickness of the nitrided layer formed using theconventional method.

FIG. 6 is a graph showing hardness distribution along the thicknessdirection of the IF steel nitrided in the KNO₃ molten salt at 640° C.for 1 hour, 2 hours, 4 hours, 8 hours and 16 hours.

As shown in the FIG. 6, for IF steel, as the nitriding time increases,the difference in hardness between the surface and the interiordecreases, resulting in the IF steel having, as well as an increasedsurface hardness, an increased bulk hardness, as a consequence ofnitrogen diffusing into the interior and the difference in concentrationthereof between the surface and the interior decreasing. In other word,the nitriding method in accordance with the present invention will leadto an IF steel having an increased surface and bulk hardness, resultingfrom a nitrogen diffusing into the interior at a higher diffusion rate.

Fifth Embodiment

In the fifth embodiment of the present invention, steel is nitridedusing the Ca(NO₃)₂ molten salt.

The steel to be nitrided in the fifth embodiment is low carbon steel.

Since Ca(NO₃)₂ is highly hygroscopic at a room temperature, includingcombined water, it is preferred to use Ca(NO₃)₂ after removing moistureby heating for a predetermined time.

The fifth embodiment of the present invention includes the process ofremoving moisture by heating Ca(NO₃)₂ for 4 hours at 100° C. to 150° C.,heating Ca(NO₃)₂ to 580° C. to form the Ca(NO₃)₂ molten-salt bath andsubmerging the low carbon steel in the bath for 3 hours.

FIG. 7 is a graph showing the surface hardness profile in low carbonsteel nitrided by the fifth embodiment of the present invention.

As shown in FIG. 7, the low carbon steel nitrided by the fifthembodiment is nitrided to a depth of 0.5 mm from the surface, and hasthe surface hardness that is 2 times higher than the surface hardness(As) of the steel before nitriding.

Sixth Embodiment

In the sixth embodiment of the present invention, steel is nitridedusing a molten mixture of KNO₃ and NaNO₃.

In the sixth embodiment of the present invention, the low carbon steelis nitrided in the molten mixture of KNO₃ and NaNO₃ whose mixture ratiosare 1:1, 8:2 and 2:8.

Table 7 shows the surface hardness values of steels nitrided by thesixth embodiment of the present invention. Various types of steel aresubmerged in the molten mixture of KNO₃ and NaNO₃ whose ratio is 1:1 for12 or 24 hours at 650° C.

At this time, the hardness is measured using a Vickers hardness testerunder a load of 3 kg.

The hardness values of the steels nitrided in the mixture of KNO₃ andNaNO₃ increase by 69% to 251% depending on the steel type.

TABLE 7 Change of Hardness (Hv) Nitriding Increasing Time Before Afterrate Type of steel (h) nitriding nitriding (%) Ultra-low 24 128 449 251carbon steel low carbon 12 194 406 109 steel Medium carbon 12 183 391114 steel High carbon 24 230 389 69 steel Alloy steel 24 226 387 71

Various steels are submerged in the mixture of KNO₃ and NaNO₃ whoseratio is 1:1 at 580° C., and changes in surface hardness and tensilestrength of the nitrided steels depending on nitriding time aremeasured.

As shown in Table 8, nitriding in accordance with the fifth embodimentof the present invention increases the hardness and the tensile strengthof all the steels. The hardness and tensile strength increase withincreasing nitriding time.

TABLE 8 Change of Hardness Change of Tensile (Hv) strength (kgf/mm²)Nitriding Increasing Increasing Type of Time Before After rate BeforeAfter rate steel (h) nitriding nitriding (%) nitriding nitriding (%)Ultra- 3 120 283 136 35 48 37 low 12 120 421 251 35 92 163 carbon steellow 3 200 283 42 45 55 22 carbon 12 200 403 102 45 79 76 steel Medium 3130 181 39 45 57 27 carbon 12 130 398 206 45 88 84 steel High 3 150 20134 60 76 27 carbon 12 150 391 161 60 87 45 steel Alloy 3 200 274 37 5575 36 steel 12 200 409 105 55 90 64

FIG. 8 is a graph showing the hardness profiles of steel nitrided at680° C. for 200 minutes in the KNO₃ bath, the NaNO₃ bath, the 50%KNO₃-50% NaNO₃ mixture bath at 680° C. for 200 minutes.

The hardness was measured using a Vickers hardness tester.

In FIG. 8, the steel nitrided in the mixture bath has a nitrided depthof 1.5 mm and a surface hardness of 160 Hv, which is higher than that ofthe steel nitrided in the single salt baths and 3 times higher than thatof the steel before nitriding.

FIG. 9 is a graph showing the hardness profiles of the low carbon steelnitrided in the 80% KNO₃-20% NaNO₃ bath and 20% KNO₃-80% NaNO₃ bath at650° C. for 4 hours, respectively.

As shown in FIG. 9, the surface hardness of the steel nitrided in themixture baths is about 2 times higher than that of the steel beforenitriding.

The present invention can solve an environmental pollution problem andcan reduce a cost for nitriding steels by using molten non-cyanidesalts, such as sodium nitrate (NaNO₃), sodium nitrite (NaNO₂), calciumnitrate (Ca(NO₃)₂) and their mixtures.

Since the present invention can increase the nitrided depth ornitrogen-diffusion depth of steels two to six times higher than thatobtained using conventional nitriding methods, thereby nitriding theinner part as well as the surface of the metal, its applications areextended to various fields.

Since the present invention can be applied to bulk hardening as well assurface hardening of steels by increasing hardness and tensile strengthof the metal, it is possible to apply the present invention to manyfields including light and highly strong automobile components anddiverse structural members which require improved wear resistance,corrosion resistance and fatigue life.

The present application contains subject matter related to Korean patentapplication No. 2006-0049077, filed in the Korean Intellectual PropertyOffice on May 30, 2006, the entire contents of which are incorporatedherein by reference.

The terms and words used in the present specification and claims shouldnot be construed to be limited to the common or dictionary meaning,because an inventor defines the concept of the terms appropriately todescribe his/her invention as best he/she can. Therefore, they should beconstrued as a meaning and concept fit to the technological concept andscope of the present invention.

Therefore, the embodiments and structure described in the presentspecification are nothing but one preferred embodiment of the presentinvention, and do not represent all of the technological concept andscope of the present invention. Therefore, it should be understood thatmany equivalents and modified embodiments that can substitute thosedescribed in this specification exist.

1. A method for nitriding a metal in a salt bath, comprising the stepsof: a) immersing at least one salt selected from a group consisting ofKNO₃, KNO₂, Ca(NO₃)₂, NaNO₃ and NaNO₂ into the salt bath; b) melting thesalt by heating and maintaining the molten salt at a predeterminedtemperature; and c) submerging the metal in the salt bath.
 2. The methodas recited in claim 1, wherein the predetermined temperature is within arange of 400° C. to 700° C.
 3. The method as recited in claim 1,wherein, in the step c), a submerging time is within a range of 1 minuteto 24 hours.
 4. The method as recited in claim 2, wherein the metal isone of iron and steels.
 5. The method as recited in claim 3, wherein themetal is one of iron and steels.
 6. A metal nitrided in a salt bathincluding at least one selected from a group consisting of KNO₃, KNO₂,Ca(NO₃)₂, NaNO₃ and NaNO₂, wherein the metal is iron and the iron isnitrided to a depth of 0.1 mm to 3.0 mm from the surface.
 7. A metalnitrided in a salt bath including at least one selected from a groupconsisting of KNO₃, KNO₂, Ca(NO₃)₂, NaNO₃ and NaNO₂, wherein the metalis steel and the steel is nitrided to a depth of 0.1 mm to 3.0 mm fromthe surface.
 8. The metal as recited in claim 7, wherein the steel is atleast one selected from a group consisting of ultra low carbon steel,low carbon steel, medium carbon steel, high carbon steel, alloy steeland IF steel.
 9. The metal as recited in claim 8, wherein the ultra lowcarbon steel has a surface hardness being more than 120 Hv to equal toor less than 450 Hv.
 10. The metal as recited in claim 8, wherein thelow carbon steel has a surface hardness being more than 200 Hv to equalto or less than 410 Hv.
 11. The metal as recited in claim 8, wherein themedium carbon steel has a surface hardness being more than 130 Hv toequal to or less than 420 Hv.
 12. The metal as recited in claim 8,wherein the high carbon steel has a surface hardness being more than 150Hv to equal to or less than 400 Hv.
 13. The metal as recited in claim 8,wherein the alloy steel has a surface hardness being more than 200 Hv toequal to or less than 410 Hv.
 14. The metal as recited in claim 8,wherein the IF steel has a surface hardness being more than 165 Hv toequal to or less than 400 Hv.
 15. The metal as recited in claim 9,wherein the ultra-low carbon steel has a tensile strength being morethan 35 kgf/mm² to equal to or less than 110 kgf/mm².
 16. The metal asrecited in claim 10, wherein the low carbon steel has a tensile strengthbeing more than 45 kgf/mm² to equal to or less than 110 kgf/mm².
 17. Themetal as recited in claim 11, wherein the medium carbon steel has atensile strength being more than 45 kgf/mm² to equal to or less than 100kgf/mm².
 18. The metal as recited in claim 12, wherein the high carbonsteel has a tensile strength being more than 60 kgf/mm² to equal to orless than 95 kgf/mm².
 19. The metal as recited in claim 13, wherein thealloy steel has a tensile strength being more than 55 kgf/mm² to equalto or less than 110 kgf/mm².
 20. The metal as recited in claim 14,wherein a chromium content of the steel ranges from 0.1 wt % to 1.5 wt%.
 21. The metal as recited in claim 15, wherein a chromium content ofthe steel ranges from 0.1 wt % to 1.5 wt %.
 22. The metal as recited inclaim 16, wherein a chromium content of the steel ranges from 0.1 wt %to 1.5 wt %.
 23. The metal as recited in claim 17, wherein a chromiumcontent of the steel ranges from 0.1 wt % to 1.5 wt %.
 24. The metal asrecited in claim 18, wherein a chromium content of the steel ranges from0.1 wt % to 1.5 wt %.
 25. The metal as recited in claim 19, wherein achromium content of the steel ranges from 0.1 wt % to 1.5 wt %.
 26. Themetal as recited in claim 14, wherein a molybdenum content of the steelranges from 0.05 wt % to 0.5 wt %.
 27. The metal as recited in claim 15,wherein a molybdenum content of the steel ranges from 0.05 wt % to 0.5wt %.
 28. The metal as recited in claim 16, wherein a molybdenum contentof the steel ranges from 0.05 wt % to 0.5 wt %.
 29. The metal as recitedin claim 17, wherein a molybdenum content of the steel ranges from 0.05wt % to 0.5 wt %.
 30. The metal as recited in claim 18, wherein amolybdenum content of the steel ranges from 0.05 wt % to 0.5 wt %. 31.The metal as recited in claim 19, wherein a molybdenum content of thesteel ranges from 0.05 wt % to 0.5 wt %.
 32. The metal as recited inclaim 14, wherein a nickel content of the steel ranges from 0.1 wt % to10 wt %.
 33. The metal as recited in claim 15, wherein a nickel contentof the steel ranges from 0.1 wt % to 10 wt %.
 34. The metal as recitedin claim 16, wherein a nickel content of the steel ranges from 0.1 wt %to 10 wt %.
 35. The metal as recited in claim 17, wherein a nickelcontent of the steel ranges from 0.1 wt % to 10 wt %.
 36. The metal asrecited in claim 18, wherein a nickel content of the steel ranges from0.1 wt % to 10 wt %.
 37. The metal as recited in claim 19, wherein anickel content of the steel ranges from 0.1 wt % to 10 wt %.
 38. Themetal as recited in claim 14, wherein a manganese content of the steelranges from 0.1 wt % to 2.0 wt %.
 39. The metal as recited in claim 15,wherein a manganese content of the steel ranges from 0.1 wt % to 2.0 wt%.
 40. The metal as recited in claim 16, wherein a manganese content ofthe steel ranges from 0.1 wt % to 2.0 wt %.
 41. The metal as recited inclaim 17, wherein a manganese content of the steel ranges from 0.1 wt %to 2.0 wt %.
 42. The metal as recited in claim 18, wherein a manganesecontent of the steel ranges from 0.1 wt % to 2.0 wt %.
 43. The metal asrecited in claim 19, wherein a manganese content of the steel rangesfrom 0.1 wt % to 2.0 wt %.
 44. The metal as recited in claim 14, whereina boron content of the steel ranges from 0.001 wt % to 0.1 wt %.
 45. Themetal as recited in claim 15, wherein a boron content of the steelranges from 0.001 wt % to 0.1 wt %.
 46. The metal as recited in claim16, wherein a boron content of the steel ranges from 0.001 wt % to 0.1wt %.
 47. The metal as recited in claim 17, wherein a boron content ofthe steel ranges from 0.001 wt % to 0.1 wt %.
 48. The metal as recitedin claim 18, wherein a boron content of the steel ranges from 0.001 wt %to 0.1 wt %.
 49. The metal as recited in claim 19, wherein a boroncontent of the steel ranges from 0.001 wt % to 0.1 wt %.
 50. The metalas recited in claim 14, wherein a titanium content of the steel rangesfrom 0.001 wt % to 0.1 wt %.
 51. The metal as recited in claim 15,wherein a titanium content of the steel ranges from 0.001 wt % to 0.1 wt%.
 52. The metal as recited in claim 16, wherein a titanium content ofthe steel ranges from 0.1 wt % to 0.1 wt %.
 53. The metal as recited inclaim 17, wherein a titanium content of the steel ranges from 0.001 wt %to 0.1 wt %.
 54. The metal as recited in claim 18, wherein a titaniumcontent of the steel ranges from 0.001 wt % to 0.1 wt %.
 55. The metalas recited in claim 19, wherein a titanium content of the steel rangesfrom 0.001 wt % to 0.1 wt %.
 56. The metal as recited in claim 14,wherein a vanadium content of the steel ranges from 0.05 wt % to 0.15 wt%.
 57. The metal as recited in claim 15, wherein a vanadium content ofthe steel ranges from 0.05 wt % to 0.15 wt %.
 58. The metal as recitedin claim 16, wherein a vanadium content of the steel ranges from 0.05 wt% to 0.15 wt %.
 59. The metal as recited in claim 17, wherein a vanadiumcontent of the steel ranges from 0.05 wt % to 0.15 wt %.
 60. The metalas recited in claim 18, wherein a vanadium content of the steel rangesfrom 0.05 wt % to 0.15 wt %.
 61. The metal as recited in claim 19,wherein a vanadium content of the steel ranges from 0.05 wt % to 0.15 wt%.
 62. The metal as recited in claim 14, wherein a niobium content ofthe steel ranges from 0.005 wt % to 0.1 wt %.
 63. The metal as recitedin claim 15, wherein a niobium content of the steel ranges from 0.005 wt% to 0.1 wt %.
 64. The metal as recited in claim 16, wherein a niobiumcontent of the steel ranges from 0.005 wt % to 0.1 wt %.
 65. The metalas recited in claim 17, wherein a niobium content of the steel rangesfrom 0.005 wt % to 0.1 wt %.
 66. The metal as recited in claim 18,wherein a niobium content of the steel ranges from 0.005 wt % to 0.1 wt%.
 67. The metal as recited in claim 19, wherein a niobium content ofthe steel ranges from 0.005 wt % to 0.1 wt %.
 68. The metal as recitedin claim 14, wherein an aluminum content of the steel ranges from 0.005wt % to 0.1 wt %.
 69. The metal as recited in claim 15, wherein analuminum content of the steel ranges from 0.005 wt % to 0.1 wt %. 70.The metal as recited in claim 16, wherein an aluminum content of thesteel ranges from 0.005 wt % to 0.1 wt %.
 71. The metal as recited inclaim 17, wherein an aluminum content of the steel ranges from 0.005 wt% to 0.1 wt %.
 72. The metal as recited in claim 18, wherein an aluminumcontent of the steel ranges from 0.005 wt % to 0.1 wt %.
 73. The metalas recited in claim 19, wherein an aluminum content of the steel rangesfrom 0.005 wt % to 0.1 wt %.
 74. The metal as recited in claim 15,wherein the ultra-low carbon steel has a carbon content ranging from atleast 0.0001 wt % to less than 0.13 wt %.
 75. The metal as recited inclaim 16, wherein the low carbon steel has a carbon content ranging fromat least 0.13 wt % to less than 0.2 wt %.
 76. The metal as recited inclaim 17, wherein the medium carbon steel has a carbon content rangingfrom at least 0.21 wt % to less than 0.51 wt %.
 77. The metal as recitedin claim 18, wherein the high carbon steel has a carbon content rangingfrom at least 0.51 wt % to less than 2.0 wt %.